Decline in physical function has a multidimensional impact in the lives of older adults and is a priority research area relative to the third of HIV- infected adults in the United States who are currently 50 years of age and older. The objective of this study is to investigate the relationship of depression and behavioral factors (alcohol, drug, and tobacco use) with baseline physical function in veterans living with HIV/AIDS compared to uninfected veterans; and to determine the effect of these factors on change in physical function over five-years of follow- up. We propose to carry out a cross-sectional study to test the hypothesis that higher levels of depression and behavioral factors will be associated with declined physical function after controlling for demographic characteristics (age, income, education, sexual orientation, and homelessness) and medical co-morbidities (e.g. cardiovascular and pulmonary disease, diabetes, hepatitis C, arthritis, and cancer). In addition, we propose a longitudinal study to test the hypothesis that depression and behavioral factors at baseline will be independently associated with change in physical function after controlling for demographic characteristics, and co-morbidities. Further, we hypothesize that, among veterans living with HIV, depression and substance abuse will decrease physical function at a greater rate than among those without these risk factors, due to the mediating effects of lower adherence to antiretroviral therapy (ART) and consequent progression in HIV disease. To test these hypotheses we will use existing data from the Veteran Aging Cohort Study (VACS). Examining physical function among HIV-infected and uninfected adults will improve our understanding of risk factors associated with physical function and will provide information for developing early interventions to delay limitations in physical function. The overall goal of this mentored career development fellowship proposal is to become a leader in the domains of psychosocial and behavioral factors relevant to physical function and HIV/AIDS. The specific goals of this fellowship are: 1) To expand my experience and knowledge in the research area of HIV and aging, with special focus in the psychological factors and substance use that impact physical function; 2) To further my training to enable me to become an independent HIV researcher with proficiency in grant writing; 3) To increase my proficiency in advanced and longitudinal analytic methods; and 4) To further develop my proficiency in the ethical conduct of research. Equipped with multidisciplinary training in aging, physical function, grant writing skills, advanced longitudinal analytic methods, and ethical conduct, I will pursue a career in research that will allow me to advance our understanding of factors affecting physical function, which ultimately lead to negative health outcomes in adults aging with HIV.